This achievement will very likely be obtained by naturally playing through the game!
If you however want to unlock this achievement very early on, a good point to farm this is in the DEAD CITY
level. About halfway trough the level you will enter a store and you will walk in slow motion for a few seconds. During this vision you will hear an alarm beeping.
In this store you will be ambushed by 5-7 Watchmen(which are basically outdoor Nosalises). Kill them all and reload checkpoint. The progress does not reset and you can farm this achievement quickly. This is also a great place for these achievements:
There are pistols and shotguns scattered around the dead city. However if you want to make 100% sure you have the right weapon load up the MARKET
level right before the dead city and buy the weapons from the vendors before heading out to the surface.